I. Field of the Invention
The present invention relates generally to waveguide to microstrip transitions of high frequency electromagnetic radiation.
II. Description of Related Art
The use of high frequency radio devices has become increasingly popular. For example, automotive radar has been allotted a frequency band at approximately 77 gigahertz.
The propagation of electromagnetic radiation at such high frequencies, however, has always presented special problems to microwave engineers. Conventional electronic circuitry cannot normally be used to propagate such high frequency signals due to the inherent capacitance and inductance present in such conventional electronic circuitry. Such capacitance and inductance usually results in unacceptable attenuation of the microwave signal.
Consequently, waveguides and microstrips are conventionally used to propagate high frequency radio signals in electronic circuits. In a waveguide, an elongated channel is formed by an electrically conductive material so that the high frequency signal travels through the interior of the conductor. Such waveguides are highly efficient for conducting high frequency signals along relatively long distances. Waveguides, however, cannot generally be used to directly drive a microwave antenna.
Microstrips are also utilized to propagate microwave energy. Such microstrips include a conductive strip on one side of a dielectric substrate and a ground plane on the opposite side of the dielectric substrate. The microwave energy is conveyed along the microstrip in between the microstrip and the ground plane Such microstrips may be directly connected to a microwave antenna to drive the antenna.
Consequently, in many applications, such as automotive radar, vehicle to satellite radio links, vehicle to base station radio links, etc., it is necessary to transition microwave energy from a waveguide to a microstrip. Such devices are known as waveguide to microstrip transitions.
There have been previously known waveguide to microstrip transitions which propagate the microwave energy from the waveguide to the microstrips. These previously known transitions typically include a dielectric substrate having a ground plane on one side and a microstrip on its opposite side. The dielectric substrate is positioned across an open end of the microwave guide so that an opening in the ground plane registers with the open end of the waveguide.
A back short is then positioned on the side of the dielectric substrate opposite from the ground plane so that the back short forms a cavity which registers with the open end of the waveguide as well as the opening formed through the ground plane. An end of the microstrip is then positioned through an opening in the back short so that the free end of the microstrip is positioned within the cavity formed by the back short.
In operation, the microwave energy from the waveguide propagates through the dielectric substrate and into the back short cavity. That electromagnetic energy then propagates out through the microstrip to another portion of the circuitry, typically a microwave antenna. These previously known waveguide to microstrip transitions, however, have all suffered from certain disadvantages.
One disadvantage of the previously known waveguide to microstrip transitions is that the microstrip must be precisely positioned within the back short cavity for proper impedance matching. Otherwise, an impedance mismatch results which in turn results in a loss of power in the waveguide to microstrip transition. However, in many manufacturing situations, such precision is difficult to obtain with consistency.
A still further disadvantage of these previously known waveguide to microstrip transitions is that they have limited bandwidth and increased return loss. Such limited bandwidth and increased return loss resulted from the resonant nature of the single microstrip probe and its location within the back short cavity.